In orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) communications systems, time-frequency resources may be shared among a number of mobile stations (MS). A base station (BS) may assign the time-frequency resources to an MS using an assignment message, which may be transmitted as part of a control channel. Ideally, information regarding an assignment of the time-frequency resources to an MS may be transmitted to the MS using as little control channel overhead (for example, fewest bits of information) as possible, while maintaining as much assignment flexibility as possible.
In a commonly used technique to minimize control channel overhead, the BS may transmit the time-frequency resource assignment in the form of an index to a channel tree. The channel tree may include channel tree nodes, wherein each channel tree node may correspond to a specific portion of the time-frequency resources of the OFDMA communications system. In general, the structure of the channel tree may dictate the permissible assignments of the time-frequency resources. For example, if there are eight time-frequency resources that may be assigned and if the channel tree has the structure of a binary tree, then an assignment may be transmitted using four bits of information.
In an alternate approach to assigning the time-frequency resources, the BS may transmit a bitmap to the MS, wherein each bit of the bitmap may represent a time-frequency resource that may be assigned and the value of the bit may represent the time-frequency resources assigned to the MS. For example, if there are eight time-frequency resources that may be assigned, then an assignment may be transmitted using eight bits of information, with one bit per time-frequency resource. Additionally, if a bit in the eight-bit sequence has a value of one (1), then a corresponding time-frequency resource may be assigned to the MS, while if a bit has a value of zero (0), then a corresponding time-frequency resource may not be assigned to the MS.